LASER-INDUCED FORMATION OF POROUS SILICON

The mechanism of photoelectrochemical porous silicon formation in fluo ride solutions under laser illumination and dark conditions has been i nvestigated. Experiments were performed in a PTFE cell with a plastic window to allow the laser path to be horizontal and the silicon electr ode to be in a vertical position inside the cell. Dark and illuminated anodic and cathodic current-voltage (I-V) curves were both measured i n real time by chopping the laser beam. N-type wafers of resistivities 0.001 to 22 Omega . cm have been investigated for various conditions of illumination intensity and polarization. We focused our attention o n relatively low illumination intensities similar to 10(-5)-10(-8) W/m m(2) and high HF concentration. By measuring the dissolution rate and the photogenerated current we estimated that the main reaction path re quires two holes for each Si atom as: Si + 2HF + 2h(+) --> SiF2 + 2H() hv. The utilization of this technique for direct projection printing of porous Si images of 10 Crm resolution was demonstrated.